Process for coating metal tubes and use of the coated tubes

ABSTRACT

A process for the coating of metal tubes by the application of polyethylene to the preheated tubes, a polyethylene with a melt index of over 1 g/10 minutes being applied first to the pre-heated metal tube and a polyethylene with a low melt index being applied thereupon, the process comprising a first step of applying a polyethylene with a melt index of 1.2 to 70 g/10 minutes (190° C./2.16 kg) to the metal tube which has been pre-heated to a temperature of at least 200° C., a second step in which the coating is cooled to a temperature of about 110° to 170° C. and a third step of applying a self-supporting film of polyethylene with a melt index of 0.1 to 7 g/10 minutes at this temperature so that the total coating has a minimum layer thickness of 1.5 to 4 mm and a coated article coated according to said process.

The coating of metal tubes with a strip of polyethylene coming directlyfrom an extruder is known. In such a process the metal tube is firstpreheated to a temperature of about 180° C. The types of polyethyleneused in the process generally have melt indices (190° C./2.16 kg) of 0.4to 0.7 g/10 minutes. A time of about 4 minutes is required to form alayer of polyethylene on the metal of about 3.5 mm thick.

The coating of metal tubes with polyethylene powder is also known andthe powder is applied e.g. by sprinkling, flinging or throwing onto thetube which is preheated to 300° to 360° C. The polyethylene types whichmay be used in this process generally have melt indices (190° C./2.16kg) of 1.2 to 1.7 g/10 minutes. In this process, the melt index of thepolyethylene must be higher than that mentioned for the above-mentionedprocess, since easier melting is essential. Depending on the tubediameter, the coating time in such powder coating processes is generallyabout 5 minutes. However, this type of process has the disadvantage thata relatively high preheating temperature, and consequently a highconsumption of energy is required to melt the polyethylene, as the meltindex should not be too low to ensure sufficient corrosion protection.

A composite metal tube coating has been described which consists of apolyethylene powder layer welded onto a steel tube together with apolyethylene layer which is welded onto the powder coating and which hasbeen wound, for example in the form of a polyethylene film from anextruder, onto the hot sintered-on polyethylene powder layer. When thefirst polyethylene layer is applied, the steel tube should have beenpreheated e.g. to 150° C. However, at this temperature it is notpossible to produce a smooth coherent layer in a desired requisiteminimum layer thickness of 1.5 to 4 mm.

In another known process, a polyethylene layer is applied to a steeltube, after first coating the tube with a layer, about 0.05 mm thick, ofan adhesion promoter which is applied at a tube temperature lyingapproximately 100° C. above the setting point of the adhesion promotor.The layer of polyethylene onto the adhesion promotor is applied at atube temperature, e.g. at 140° C., lying 20° to 50° C. above the settingpoint. The adhesion promotor of the first layer can be applied in powderform or by winding a film round the tube. The application of the secondlayer may be effected by pre-extrusion in the form of a double tubing orof a double winding film. In this process, also, the temperature of 140°C. is not sufficient for the purposes desired according to the presentinvention, namely for a requisite minimum layer thickness.

The covering of a steel tube surface with a polyethylene having a highmelt index, for example 1 to 1.5 g/10 minutes (ASTM-D 12 38-53T) hasalso been disclosed. A second layer of a polyethylene with a low meltindex, for example 0.2 to 0.5 g/10 minutes, is applied to the covering.Both layers are applied in the form of a polyethylene powder. If twolayers are applied on top of one another in the form of powder, thesurface sometimes leaves much to be desired in respect of itshomogeneity and its smoothness.

A process with a smaller consumption of energy and time is thereforedesired which would give products with properties as good as those fromthe known processes.

The present invention provides a process for coating metal tubes by theapplication of polyethylene to the preheated tubes, a polyethylene witha melt index of over 1 g/10 minutes being applied first to the preheatedmetal tube and a polyethylene with a low melt index being appliedthereupon, said process being characterised in that in a first step apolyethylene with a melt index of 1.2 to 70, advantageously 15 to 70,and preferably 17 to 25 g/10 minutes (190° C./2.16 kg) is first appliedto the metal tubes which have been preheated to a temperature of atleast 200° C., whereafter in a second step the coating is cooled to atemperature of about 110° to 170° C., advantageously 110 to 50, andpreferably to 120° C., and in a third step a self-supporting film ofpolyethylene having a melt index of 0.1 to 7 g/10 minutes is thereuponapplied at this temperature, so that the total coating has a minimumthickness of 1.5 and to 4 mm.

One embodiment of the invention provides that a polyethylene powder witha melt index of about 1.2 to 1.7 g/10 minutes or a tape of polyethylenecoming directly from an extruder and having a melt index of not morethan 1.7 g/10 minutes (190° C./2.16 kg) is first applied to the metaltube which has been preheated to a temperature of at least 300° C. inthe case of powder coating and at most 250° C. in the case of coatingwith extruded tape, the coating is thereafter cooled in the second stepto a temperature of about 110 to 170, preferably 110° to 150° C., and ina third step a self-supporting photo-stabilised light-coloured film ofpolyethylene with a melt index of 0.4 to 1.1 g/10 minutes is thereuponapplied at this temperature.

Examples of photo-stabilisers for the light-coloured polyethylene filmare e.g. compounds of the benzotriazole type.

The minimum layer thickness of 1.5 to 4 mm is necessary to ensuresufficient corrosion protection for the metal tube. In many cases thelayer thickness may also be more than 4 mm.

The process according to the invention has the advantage that itsurprisingly entails a high saving of energy compared with knownprocesses and, nevertheless, a considerably higher working speed with atleast equally good product properties. Moreover, a light-colouredcoating layer provides good protection of the tubes against strongheating during any lengthy storage in the open air under strong thermalaction, for example from solar radiation, or when laid in stronglyheated soil strata. The coating from the extruder may be effected verysimply and in a time-saving manner without additional expenditure onapparatus. The tape coming from the extruder advantageously has a meltindex of at least 0.4 g/10 minutes.

Generally, in powder coating, the polyethylene for the first step has aparticle size of 1 to 600 μm, preferably 100 to 400 μm. Theself-supporting polyethylene film is applied advantageously in the formof a polyethylene tape for example of polyethylene with a melt index of0.1 to 1.2 g/10 minutes. The application may be effected by rotating thetube. This affords the advantage that the tape can be woundautomatically. The tape width can be varied as desired. It may beconveniently 10 to 1500 mm e.g. at least 20 mm. In general, a tape widthup to about 1 m is used. During application it is necessary to ensurethat the individual coil layers overlap or that the individual coillayers are simultaneously mutually joined together, in order to achievesatisfactory corrosion protection. The layer thickness of the tapes isusually 100 to 400 μm, preferably 100 to 200 μm. Light-coloured tapesare preferably white. Depending on the required end use, however,another colour may also be chosen, for example the warning colouryellow, and also light orange, light blue, light green or the like. Inthis way, the light-coloured tapes may also serve to identify the tubes.

The speed of coating may vary over wide limits. It depends on thedesired layer thickness and on the tube diameter; the outside tubediameter may be, for example, from 50 to 2000 mm. For coating a lengthof 12 m of a tube with an outside diameter of 1500 mm and a coatinglayer thickness of 3.5 mm e.g. about 15 to 45, generally about 30minutes are required in the process of the invention. For coating a tube400 mm in diameter with a layer thickness of 1.5 mm (this is the minimumthickness for sufficient corrosion protection), generally about 8 to 20,mostly about 15 minutes are required for a coating 12 m long.

In order to further improve the adhesion of the layer applied in thefirst step to the tube substrate, it is sometimes appropriate to admixwith the polyethylene powder an additional resin in the form of apolymer, e.g. polyvinyl acetate, ethylene-vinyl acetate copolymer,ethylene-acrylic acid and/or acrylate copolymer, optionally with furthercomonomers, or other polymers, advantageously in a proportion of 5 to15, preferably 5 to 10% by weight relative to the polyethylene powder.According to another embodiment of the invention it is also possible toapply such polymers to the tube before the polyethylene powder isapplied. This coating may be effected according to the conventionalcoating processes, e.g. by spraying, but preferably by powder coating.The same additional resins may also be present in the polyethylene tapein a proportion of 2 to 5% by weight relative to the polyethylene.

The requirements regarding minimum layer thickness, freedom from pores,resistance to paring, impact strength, indentation resistance,elongation at break, specific sheath resistance and ageing due to heatand light in accordance with the provisions of DIN 30670 are fullysatisfied by the coatings made according to the invention.

The tubes coated by the process according to the invention have avariety of uses. Owing to the surface protection which they provide theyare suitable, above all, for laid pipes, e.g. in pipelines for conveyingpetroleum, and also gaseous or other liquid substances or substances ofhigher viscosity, e.g. natural gas, water, settling sludge, concrete,waste waters, suspensions or the like.

Especially advantageous is the use of the tubes coated according to theinvention for laying in warm or hot areas, e.g. in desert regions.

EXAMPLES

(1) An iron tube (outside diameter 108 mm, wall thickness 10 mm ispreheated to 220° C. and then coated over 2 minutes with polyethylenepowder (melt index 17 to 25) in a layer thickness of 2 mm. After 4minutes the covering has melted and the tube temperature has dropped to160° C. Starting at this tube temperature, a polyethylene tape 110 μmthick and 50 mm wide with a melt index of 1.2 is applied at 160° C. in alayer thickness of 110 μm. The tube is then cooled to room temperatureeither by merely allowing it to stand or by passing a cooling mediumthrough the tube. Immediately after the film has been applied a perfectand smooth fusing of both covering layers takes place. After 30 minutesfrom the start of the powder coating the tube has cooled to 60° C. bybeing allowed to stand without additional cooling. (2) An iron tube(outside diameter 90 mm, wall thickness 4.5 mm) is preheated to 250° C.and is then coated over 11/2 minutes with polyethylene powder (meltindex 17 to 25) in a layer thickness of 2 mm. After 3 minutes thecoating has melted smooth and cooled to 150° C. At this tube temperaturea polyethylene tape 40 mm wide with a melt index of 1.2 and a layerthickness of 110 μm is applied and the tube is then cooled to roomtemperature. The tape is applied by being wound round the tube which isrotated about its axis, the coil being moved along the tube. Immediatelyafter the tape has been applied a perfect fusing with the firstpolyethylene layer takes place. After 12 minutes from the start of thetube coating the tube has cooled to 60° C. by being allowed to standwithout additional cooling.

(1C) (Comparison--state of the art--preheating temperature above 300°C.)

The same tube as in Example 1 is used, but with a preheating temperatureof 310° C. The tube is coated with a polyethylene powder having a meltindex of 1.2 to 1.7 g/10 minutes in a layer thickness of 2 m. After 10minutes the coating has melted smooth and has reached a temperature of180° C. The time of cooling to 60° C. by merely allowing the tube tostand is 50 minutes.

(2C) (Comparison--state of the art--preheating temperature above 300°C.)

The same tube as in Example 2 is used, but with a preheating temperatureof 360° C. The tube is coated at this temperature with a polyethylenepowder having a melt index of 1.2 to 1.7 over 11/2 minutes in a layerthickness of 2 m. After 4 minutes the covering has melted andsimultaneously cooled to 310° C. However, further heating is necessaryfor melting smooth. The tube is therefore further heated for 1 minuteafter the expiration of these 4 minutes. The covering is thus smoothafter this further minute. The time of cooling to 60° C. from the startof the powder coating and without additional cooling is 42 minutes.

As a comparison between the Examples according to the invention and thecomparative Examples according to the state of the art shows, theconsumption of energy, that is the preheating temperature andsimultaneously also the cooling time, is substantially smaller in theExamples according to the invention than in the comparative Examples.

(3) Work is carried out as in Example 1, but, instead of thepolyethylene powder, a powder in the form of a mixture of polyethylenewith 10% by weight, relative to the polyethylene, of a vinyl acetatehomopolymer is applied. A coating is obtained with properties as good asthose according to Example 1 and with a perfect smooth surface.

(4) Work is carried out as in Example 2, but, instead of thepolyethylene tape, a tape consisting of a mixture of polyethylene and3.5% by weight, relative to the polyethylene, of a vinyl acetatehomopolymer is used. A smooth and perfect coating is obtained withproperties as good as those according to Example 2.

(5) An iron tube (outside diameter 108 mm, wall thickness 10 mm) ispreheated to 310° C. and then coated over 2 minutes with polyethylenepowder (melt index 1.2 to 1.7 g/10 minutes) in a layer thickness of 2mm. After 20 minutes the coating had melted and the tube temperature haddropped to 160° C. Starting at this tube temperature a polyethylene tape200 μm thick and 50 mm wide with a melt index of 0.4 g/10 minutes isapplied at 160° C. in a layer thickness of 200 μm. The tube is thencooled to room temperature either by merely allowing it to stand or bypassing it through a cooling medium. Immediately after the tape has beenapplied a perfect and smooth fusing of both coating layers takes place.After 40 minutes from the start of the powder coating the tube hascooled to 60° C. by being allowed to stand without additional cooling.

(6) An iron tube (outside diameter 90 mm, wall thickness 4.5 mm) ispreheated to 360° C. and then coated over 11/2 minutes with polyethylenepowder (melt index 1.2 to 1.7 g/10 minutes) in a layer thickness of 2mm. After at most 8 minutes, the coating has melted smooth and after 12minutes cooled to 150° C. At this tube temperature a polyethylene tape40 mm wide with a melt index of 0.4 g/10 minutes is applied in a layerthickness of 200 μm and the tube is then cooled to room temperature. Thetape is applied by being wound round the tube which is rotated about itsaxis, the coil being moved along the tube. Immediately after the tapehas been applied a perfect fusing with the first polyethylene layertakes place. After 30 minutes from the start of the tube coating thetube has cooled to 60° C. by being allowed to stand without additionalcooling.

(7) An iron tube (outside diameter 500 mm, wall thickness 6 mm) ispreheated to 250° C. and coated with an ethylene-acrylic acid copolymerin powder form as an adhesive primer in a layer thickness of 100 μm. Apolyethylene tape coming directly from an extruder and having a meltindex of 1.2 g/10 mm and a layer thickness of 250 μm is applied to thisstill hot tube covered in this way. The winding operation is continueduntil the desired layer thickness of 4 mm is obtained. The tube iscooled to about 140° C. by merely being allowed to stand. At thistemperature a yellow polyethylene tape with a melt index of 0.7 g/10minutes, a width of 300 mm and a layer thickness of 200 μm is applied. Aperfect and smooth amalgamation of the two polyethylene covering layerstakes place. After 30 minutes from the start of the extruder coating thetube has cooled to 60° C. by being allowed to stand without additionalcooling. It is not intended that the examples given herein should beconstrued to limit the invention thereto, but rather they are submittedto illustrate some of the specific embodiments of the invention. Resortmay be had to various modifications and variations of the presentinvention without departing from the spirit of the discovery or thescope of the appended claims.

What is claimed is:
 1. A process for coating preheated metal tubes withpolyethylene in at least two steps, wherein a metal tube which has beenpreheated to a temperature of at least 200° C. and at most 250° C. iscoated in a first step with polyethylene having a melt index between 1.2and 70 g/10 min. (190° C./2.16 kg), the coating is cooled in a secondstep to a temperature between 110° and 170° C., and subsequently in athird step a self-supporting film of polyethylene having a melt indexbetween 0.1 and 7 g/10 min is applied by winding round the tube at atemperature between 110° and 170° C. so that the total coating has aminimum thickness of 1.5 to 4 mm.
 2. A process as claimed in claim 1,wherein a polyethylene having a melt index between 15 and 70 g/10 min(190° C./2.16 kg) is applied in the first step.
 3. A process as claimedin claim 1, wherein in the first step a polyethylene powder having amelt index between about 1.2 and 1.7 g/10 min (190° C./2.16 kg) andcoming directly from an extruder is applied to the metal tube which hasbeen preheated to a temperature of at least 300° C. in the case ofpowder-coating and wherein in a third step a self-supportingphoto-stabilized light-colored film of polyethylene having a melt indexbetween 0.4 and 1.1 g/10 min is applied as a coating.
 4. A process asclaimed in claim 1, wherein in the second step the coating is cooled toa temperature between 110° and 150° C.
 5. A process as claimed in claim1, wherein in the third step the polyethylene with the lower melt indexbetween 0.1 and 1.2 g/10 min is applied in the form of a polyethylenetape.
 6. A process as claimed in claim 5, wherein the polyethylene tapeis applied to a rotating tube.
 7. A process as claimed in claim 1,wherein the polyethylene coating is applied in the form of a whitepolyethylene tape having a thickness between 100 and 200 μm.
 8. Aprocess as claimed in claim 1, wherein in a first step the polyethyleneis applied in admixture with a vinyl acetate homo- or copolymer, theproportion of which being between 5 and 10% by weight, referred topolyethylene.
 9. A coated article in the form of a metal tube havingbeen coated according to the process as claimed in claim
 1. 10. A coatedarticle as claimed in claim 9 in the form of a pipeline for thetransport of petroleum.
 11. A process as claimed in claim 1, wherein inthe first step a tape of polyethylene having a melt index of at most 1.7g/10 min and coming directly from an extruder is applied to the metaltube which has been preheated to a temperature of at most 250° C. andwherein in a third step a self-supporting photo-stabilized light-coloredfilm of polyethylene having a melt index between 0.4 and 1.1 g/10 min isapplied as a coating.